A large number of applications and potential applications exist for imaging systems such as projection displays that are used to display information. Such applications include, but are not limited to, general indoor signage (e.g. shopping malls, arcades, etc.), transportation signage (e.g. arrival/departure times, etc.), in-lobby signage for office buildings, control rooms, restaurants signage, etc.
It is known to provide large displays for signage and the like by assembling a multiplicity of individual display units in an array (see, for example, United States Patent Publication No. 2008/0284675, the contents of which are incorporated herein by reference). The construction of each individual display unit may include a chassis for housing projection lamps, electronic circuits, etc., and a rear projection screen. Typically, the chassis is metallic whereas the rear projection screen is plastic; resulting in a mismatch of thermal expansion coefficients (i.e. the screen expands at a faster rate than the chassis). Therefore, in order to build an array of display units capable of operation over a wide range of environmental temperatures, the thermal expansion coefficient mismatch must be accounted for.
One method of accounting for thermal expansion is to undersize the screen so that at elevated temperatures the screen does not exceed the chassis size and cause interference with a neighbouring display unit. However, this solution is not desirable since it results in large gaps between adjacent display unit screens at nominal temperatures, in order to accommodate thermal expansion at elevated temperatures. Large gaps between adjacent screens have the potential to interfere with the optical transition from one display to the next, thereby reducing overall image quality. Another, less optimal alternative is to allow minor interference collisions within predetermine tolerance limits that do not cause damage to either the screen or chassis. However, it is difficult to manufacture display units with sufficiently high tolerance limits as to avoid damage caused by interference between adjacent units at elevated temperatures and gaps between units at nominal temperatures.
Yet another solution is to fabricate the chassis and display screen from the same material so that the chassis and screen exhibit similar thermal expansion characteristics. To achieve this, the chassis may be constructed of plastic having a comparable coefficient of thermal expansion (CTE) to the screen assembly. As the screen expands, so too does the chassis, thereby maintaining the expansion differential to a minimum. Unfortunately, a plastic chassis has potentially poor performance with respect to dimensional stability, particularly as it relates to component positioning. For example, a plastic chassis may twist and distort as it expands and contracts, resulting in misalignment of the optical components.
A skilled person in the art will understand that the drawings are for illustrative purposes only. The drawings are not intended to limit the scope of the applicant's teachings in any way.